Electroacoustic transducers, which convert electrical energy into sound energy and vice versa, have been known for decades. They are useful for various purposes, including telecommunications equipment and hearing instruments, such as hearing aids (or generally “listening devices”) that are located outside the ear or that fit within the ear canal. A conventional listening device includes a miniature microphone that receives acoustic waves and converts the acoustic waves to an audio signal. That audio signal is then processed (e.g, amplified) and sent to the receiver of the hearing aid or listening device The receiver then converts the processed signal to an acoustic signal that is broadcast towards the eardrum.
In addition to the size of the microphone becoming smaller, the amount of electronic equipment fitted with a microphone is rapidly growing. For many applications, including telecommunications equipment and hearing instruments, there is a demand for low cost, high performance miniature microphones.
The state-of-the-art transducer type used for these applications is the electret microphone. Generally, the construction of the electret microphone consists of three parts a microphone housing, an impedance conversion circuit, and the microphone cartridge. Such a cartridge typically consists of a rigid plate (“backplate”) and a metallized polymer diaphragm separated by a thin spacer. The diaphragm and the backplate form the plates of a variable capacitor. A dielectric layer in either the backplate or diaphragm is permanently electrically charged (“electret”). An incoming sound wave causes the diaphragm to move relative to the backplate, resulting in a capacitance change. The change in capacitance alters the electrical field strength from the fixed charge and, thus, causes a voltage change across the capacitor.
Because the capacitance between the backplate and diaphragm is very small, the impedance of the electrical signal source is very high, and the signal is very sensitive to interference. Therefore, electret microphones typically include an internal buffer amplifier (or impedance conversion circuit). Both the backplate and the diaphragm of the cartridge must be connected with the buffer amplifier. Generally, either the backplate or the diaphragm (“reference”) is connected with the amplifier via the microphone housing. The other (“signal”) is connected with the amplifier by a wire, which requires the use of a soldering process and/or a small drop of electrically conductive adhesive. This is often a manual step subject to human error and is time consuming.
As such, a need exists for a new type of electroacoustic transducer that is easier to manufacture and requires less parts.